The invention relates generally to heat engines and more particularly to traveling wave thermoacoustic heat engines.
One application for thermoacoustic heat engines is powering satellites, particularly in deep space. While Earth-orbiting satellites typically use solar cells as an energy source, this is not available as satellites move farther from the Sun. Consequently, these satellites are powered by a radio-isotope heat source such as plutonium whose heat is converted into electricity via the thermoelectric effect. However, this system for generating electricity is not very efficient.
Thermoacoustic heat engines are a more efficient way to generate power from a heat source. In essence, a thermoacoustic heat engine consists of a tube filled with a gas. Applying heat at one end of the tube creates a heat differential along the length of the tube and induces sound waves which can be used to convert the heat into mechanical energy. Thermoacoustic heat engines can generally be classified as either resonant or traveling wave types.
A schematic of a representative traveling wave thermoacoustic heat engine is shown in FIG. 1. A sealed system filled with, for example, pressurized helium gas, includes a torus 100, a resonator 102 and a variable acoustic load 104. Torus 100 includes cold heat exchanger 106, regenerator 108 and hot heat exchanger 110. A sound wave is induced in the helium gas by creating a temperature difference across regenerator 108 by using cold heat exchanger 106 and hot heat exchanger 110. Thermal buffer tube 112 provides a thermal buffer between hot heat exchanger 110 and the cold side (114 and 116) by providing space for the heated helium gas to oscillate without reaching the cold side. A flow straightener and heat exchanger 114 suppress prevent certain types of gas flow and reduces heat loss thereby improving the thermal efficiency of the heat engine.
Torus 100 also includes a feedback inertance 116 which provides a path for the helium gas to flow through to compliance 118, jet pump 120 and finally back to cold heat exchanger 106. The configuration and volume of inertance 116 and compliance 118 are selected to control the phase of the traveling wave induced in the helium gas. Jet pump 120 is used to reduce gas streaming and thereby improve thermal efficiency.
FIG. 1 depicts a single stage thermoacoustic heat engine. The amount of acoustic power output per acoustic power input (the gain) per stage of a thermoacoustic heat engine is limited by the temperature ratio between the hot and cold ends of the regenerator. To increase the overall gain of the engine, single stage thermoacoustic heat engines are sometimes connected in series to form a multi-stage heat engine. However, prior art multi-stage engines are thermally and mechanically cumbersome, volumetrically inefficient, do not scale down well and are subject to high thermal stresses. This is due to the geometries used to expose the gaseous working fluid's acoustic power path multiple times to a common set of thermal interfaces. A folded loop topology has been used to provide a common set of thermal interface points, but it is very large and only works for a subclass of traveling wave heat engines.
Thus, a need exists for a multi-stage thermoacoustic engine with improved volumetric and thermal efficiency, better scalability and greater resistance to high thermal stresses.